Linux – intel-ucode not updating? Firmware Bug: TSC_DEADLINE disabled due to Errata; please update microcode to version (or later)

I am getting this error on booting up:

[Firmware Bug]: TSC_DEADLINE disabled due to Errata; please update microcode to version (or later)

I see a possible mismatch between the firmware version in /boot/intel-ucode.img
and the version reported by dmesg. However, I do not know how to resolve the issue.

Kernel / distro:

# uname -a
Linux workstation 4.13.9-1-ARCH #1 SMP PREEMPT Sun Oct 22 09:07:32 CEST 2017 x86_64 GNU/Linux

CPU info (machine is a dual Xeon, each CPU has 10 cores, only 1 is listed)

# cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 79
model name      : Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz
stepping        : 1
microcode       : 0xb00001c
cpu MHz         : 2200.126
cache size      : 25600 KB
physical id     : 0
siblings        : 20
core id         : 0
cpu cores       : 10
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 20
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 intel_ppin intel_pt tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm rdt_a rdseed adx smap xsaveopt cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts
bugs            :
bogomips        : 4402.59
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

This entry shows that I'm using the required two initrd lines with intel-ucode.img listed first:

# cat /boot/loader/entries/arch.conf 
title          Arch Linux
linux          /vmlinuz-linux
initrd         /intel-ucode.img
initrd         /initramfs-linux.img
options  ...

Confirms I'm using the above entry:

# cat /boot/loader/loader.conf 
default arch

Confirms my intel-ucode.img file has the correct date and is in the correct location:

# ls -la /boot/
total 137324
drwxr-xr-x 5 root root     4096 Nov  7 02:18 .
drwxr-xr-x 1 root root      224 Oct 29 02:38 ..
drwxr-xr-x 4 root root     4096 May 16  2016 EFI
-rwxr-xr-x 1 root root 35074017 Nov  2 05:58 initramfs-linux-fallback.img
-rwxr-xr-x 1 root root 19212497 Nov  2 05:58 initramfs-linux.img
-rwxr-xr-x 1 root root 27503970 May 19  2016 initramfs-linux-lts-fallback.img
-rwxr-xr-x 1 root root 19029219 Nov  2 05:58 initramfs-linux-lts.img
-rwxr-xr-x 1 root root 21556194 May 29  2016 initramfs-linux-simple-fallback.img
-rwxr-xr-x 1 root root  6569920 May 29  2016 initramfs-linux-simple.img
-rwxr-xr-x 1 root root  1303040 Jul  9 08:15 intel-ucode.img
drwxr-xr-x 3 root root     4096 Jun 24  2016 loader
-rwxr-xr-x 1 root root  5502736 Oct 22 03:09 vmlinuz-linux
-rwxr-xr-x 1 root root  4827424 Oct 30 04:20 vmlinuz-linux-lts

dmesg indicates microcode version 0xb000020 (or later) is required and that my version is 0xb00001c

# dmesg | grep microcode
[    0.000000] microcode: microcode updated early to revision 0xb00001c, date = 2016-05-20
[    0.000000] [Firmware Bug]: TSC_DEADLINE disabled due to Errata; please update microcode to version: 0xb000020 (or later)
[   22.175583] microcode: sig=0x406f1, pf=0x1, revision=0xb00001c
[   22.177119] microcode: Microcode Update Driver: v2.2.

Seems to confirm that my microcode version is 0xb00001c

$ grep microcode /proc/cpuinfo
microcode       : 0xb00001c

However, iucode_tool seems to indicate that my file /boot/intel-ucode.img contains microcode rev 0xb000021.

bsdtar -Oxf /boot/intel-ucode.img | iucode_tool -tb -lS - 
iucode_tool: system has processor(s) with signature 0x000406f1
microcode bundle 1: (stdin)
selected microcodes:
001/139: sig 0x000406f1, pf_mask 0xef, 2017-03-01, rev 0xb000021, size 26624

The question is, why is microcode rev 0xb000021 not be used and how can I get it "loaded" properly?

UPDATE – a bit more info as requested by jasonwryan. However, between initially asking this question and providing this update, I ran pacman -Syu and got a new kernel. So here is the requested info plus current output of uname -a

# pacman -Q linux intel-ucode
linux 4.13.11-1
intel-ucode 20170707-1

# uname -a
Linux workstation 4.13.11-1-ARCH #1 SMP PREEMPT Thu Nov 2 10:25:56 CET 2017 x86_64 GNU/Linux